Since a single crystal of metal fluoride such as calcium fluoride or barium fluoride has a high transmittance over a waveband within a wide range, and has a low dispersion and is also excellent in a chemical stability, it has been demanded greatly as an optical material such as lenses and window materials of various apparatuses using a laser emitting an ultraviolet ray or a vacuum ultraviolet ray, a camera, a CVD device and the like. In particular, the calcium fluoride single crystal has been expected as a window material of a light source, a light source system lens and a projection system lens in an ArF laser (193 nm) or an F2 laser (157 nm) which has been developed as a next generation short wavelength light source in an optical lithographic technique.
Conventionally, the metal fluoride single crystal is generally produced by a crucible depression method (the Bridgman's method) or a single crystal pulling method (the Czochralski method). The crucible depression method implies a method of gradually bringing down a melting solution of single crystal producing material in a crucible together with the crucible and cooling them, thereby growing a single crystal in the crucible. On the other hand, the single crystal pulling method implies a method of causing a seed crystal formed by a single crystal to be a target to come in contact with a surface of a melting solution of the single crystal producing material in the crucible, and subsequently, gradually pulling the seed crystal upward from a heating area of the crucible and carrying out cooling, thereby growing the single crystal below the seed crystal.
There has been a problem in that an as-grown single crystal of metal fluoride produced by the crucible depression method or the single crystal pulling method has a large number of internal defects found as shiny grains scattering a light, that is, so-called scattering bodies, through an observation under a light concentrating illumination. For example, it has been reported that a part of a lower portion of the single crystal having a small number of scattering bodies is to be selected and cut out of the whole as-grown single crystal in order to obtain an optical member having at least 160 scattering bodies in a maximum diameter of 20 μm or less per cm3 in case of the metal fluoride single crystal obtained by the crucible depression method (see Patent Document 1). Moreover, most of the scattering bodies are holes in actual situations as will be described below. In a material melting solution accommodated in the crucible, therefore, they are easily formed in a process for crystallizing an upper liquid in place of a lower liquid. In the case in which the single crystal is produced by the single crystal pulling method, they tend to be formed much more easily than in the crucible depression method.
In addition, in both of the methods, the scattering bodies are generated more remarkably in the case in which a single crystal having a large diameter is produced in place of a single crystal having a small diameter.
When a large number of scattering bodies are present in the single crystal, there is a possibility that a transmittance might be reduced by the scatter of a light, a contrast might be lowered or a flare or a ghost might be generated in the case in which the single crystal is processed into an optical material. Accordingly, it is necessary to decrease the scattering bodies in the single crystal as greatly as possible. In the crucible depression method and the single crystal pulling method, however, there has not been known a method of effectively suppressing the formation of the scattering bodies in a straight barrel portion to be the most useful place in the cut-out of the optical material or over the whole as-grown single crystal. Under the existing circumstances, there is only means for selecting and cutting out a slight portion having a small amount of the formation. Accordingly, it is hard to cut out the optical material having a large diameter. Referring to the optical material having a small diameter, moreover, most of the as-grown single crystal other than the cut portion is to be a defective product. Thus, a yield of a product is considerably low.
In such a background, the present inventors found that it is possible to greatly suppress the formation of the scattering bodies by setting a depth of a raw metal fluoride melting solution to be equal to or less than 0.65 times as great as a diameter of the straight barrel portion of the single crystal even in the single crystal pulling method in which the scattering bodies should be generally formed more greatly, and previously filed the patent application (Japanese Patent Application No. 2004-309430). According to this method, it is possible to greatly reduce a natural convection of the melting solution in the crucible which causes the formation of the scattering bodies. As a result, it is possible to efficiently produce the as-grown single crystal of the metal fluoride which has a small amount of presence of the scattering bodies and a large diameter.
In the case in which this method is to be executed by using an existing pulling apparatus which is known for producing the metal fluoride single crystal, however, it is necessary to deeply accommodate the melting solution by all means in the beginning of the start of pulling even if the raw metal fluoride is accommodated as slightly as possible in a crucible having a deep bottom and a large diameter in order to execute the pulling. Therefore, the effects cannot be exhibited sufficiently.
In the case in which the single crystal to be pulled has a large diameter and a straight barrel portion has a great length, particularly, there is a possibility that the melting solution might be accommodated more deeply than the specified value in the beginning of the start of the pulling. In this case, the growth of the single crystal in a state in which the melting solution is shallow can be achieved only in such a condition that the pulling is considerably advanced. In an as-grown single crystal thus obtained, a considerable number of scattering bodies are formed in an upper part such as a shoulder portion.
As a method of maintaining a certain depth of the melting solution from the beginning of the start of the pulling of the metal fluoride single crystal to the completion of the pulling, there has been known a method of supplementing a material corresponding to the melting solution decreased by the pulling of the single crystal into the crucible. However, a reactivity of the metal fluoride to oxygen or water at a high temperature is very high. For this reason, it is necessary to carry out a careful purity increasing treatment such as a high temperature dehydrating treatment or a fluorinating treatment for the raw material to be supplemented. In respect of the structure of the apparatus, it is very hard to supplement the raw material in a state in which a high purity is thus maintained.
For the apparatus for pulling a single crystal, it has been known that a crucible having a double structure constituted by an inner crucible and an outer crucible is used to enhance a uniformity of an impurity concentration for producing a semiconductor single crystal such as silicon doped with an impurity (Patent Document 2 and Patent Document 3). However, the pulling apparatus comprising the double structured crucible which has been described in these documents is used for a single crystal growth of a semiconductor material, and there is no motive of use in the production of a metal fluoride single crystal in which an impurity is not doped. Actually, an example in which the apparatus is used has not been known at all.
Patent Document 1: International Laid-Open No. 02/077676 pamphlet
Patent Document 2: Japanese Patent Application No. Sho 61-261288
Patent Document 3: Japanese Patent Application No. Sho 62-87489